Argon‐40 in Lunar Exosphere: Observations From CHACE‐2 on Chandrayaan‐2 Orbiter

Abstract We report the first observation of Argon‐40 (Ar‐40) in the mid latitude regions ( to ) of the lunar exosphere from CHandra’s Atmospheric Composition Explorer‐2 (CHACE‐2) experiment aboard Chandrayaan‐2 orbiter. The number density of Ar‐40 shows pre‐sunrise, sunrise and sunset peaks as well as nightside minima, typical of a condensable gas, which is similar to the features seen at the low latitudes in previous observations. The CHACE‐2 observed number densities of Ar‐40 and its diurnal variation at low latitudes ( to ) is consistent with LACE/Apollo observations. CHACE‐2 observations show Ar‐40 enhancements over certain longitude sectors. In addition to KREEP region, Ar‐40 bulges are observed at other longitudes, including the South Pole Aitken (SPA) terrain. The global distribution of Ar‐40 shows that the sunrise peak is observed at the same local time over highlands and mare regions. These observations call for a deeper understanding of the surface‐exosphere interactions and source distribution.